Steroid precursors, or pro-hormones, having a sulphate group in the 3-position of the steroid nucleus, referred to hereinafter simply as steroid sulphates, are known to play an important part as intermediates in steroid metabolism in the human body. Oestrone sulphate and dehydroepiandrosterone (DHA) sulphate, for example, are known to play an important role as intermediates in the production, in the body, of oestrogens, such as oestrone and oestradiol. Oestrone sulphate, in particular, is known, for example, to represent one of the major circulating oestrogen precursors particularly in post-menopausal women and oestrone sulphatase activity in breast tumours is 100-1000 fold greater than that of other enzymes involved in oestrogen formation (James et al., Steroids, 50, 269-279 (1987)).
Not only that, but oestrogens such as oestrone and oestradiol, particularly the over-production thereof, are strongly implicated in malignant conditions, such as breast cancer, see Breast Cancer, Treatment and Prognosis: Ed. R. A. Stoll, pp. 156-172, Blackwell Scientific Publications (1986), and the control of oestrogen production is the specific target of many anti-cancer therapies, both chemotherapy and surgical, e.g. oophorectomy and adrenalectomy. So far as endocrine therapy is concerned, efforts have so far tended to concentrate on aromatase inhibitors, i.e. compounds which inhibit aromatase activity, which activity is involved, as the accompanying oestrogen metabolic flow diagram (FIG. 1) shows, in the conversion of androgens such as androstenedione and testosterone to oestrone and oestradiol respectively.
In recently published International Application WO91/13083 a proposal has been made to target a different point in the oestrogen metabolic pathway, or rather two different points, that is to say the conversion of DHA sulphate and oestrone sulphate to DHA and oestrone, respectively, by steroid sulphatase activity, and using 3-monoalkylthiophosphonate steroid esters as a steroid sulphatase inhibitor, more especially oestrone-3-monomethylthiophosphonate.